On the thermo-elastic behaviour of kyanite: a neutron powder diffraction study up to 1200°C

The high-temperature (HT) behaviour of kyanite (Al₂SiO₅) was investigated by in situ neutron powder diffraction up to 1200°C. Within the investigated T range, no phase transition was observed. The axial and volume thermal expansion coefficient (α_j = l_j⁻¹(∂l_j/∂T), α_V = V⁻¹ (∂V/∂T)), calculated by weighted linear regression through the data points, are: α_a = 5.5(2)×10⁻⁵, α_b = 5.9(2)×10⁻⁵, α_c = 5.18(8)×10⁻⁵, α_V = 7.4(1)×10⁻³ °C⁻¹, with α_a:α_b:α_c = 1.06:1.14:1. All three angles of the kyanite lattice show a slight decrease with T, with ∂α/∂T = −2(2)×10⁻⁵, ∂β/∂T = −4(1)×10⁻⁵, ∂γ/∂T = −10(2)×10⁻⁵°/°C. The magnitudes of the principal Lagrangian unit-strain coefficients (ε₁, ε₂, ε₃) and the orientations of the thermal strain-ellipsoids, between the ambient temperature and each measured T, were calculated. The magnitude and the orientation of all the three unit-strain coefficients are almost maintained constant with T. At T-T₀ = 1177°C, ε₁∧a = 76(2)°, ε₁∧b = 70(2)°, ε₁∧c = 38(3)°, ε₂∧a = 49(3)°, ε₂∧b = 66(3)°, ε₂∧c = 127(4)°, ε₃∧a = 135(3)°, ε₃∧b = 31(3)°, ε₃∧c = 91(2)° with ε₁: ε₂: ε₃ = 1.57:1.29:1. The structural refinements, performed at 23, 600, 650, 700, 750, 800, 900, 950, 1050 and 1200°C allowed the description of the structural evolution and the main T-induced deformation mechanisms, which are mainly represented by the polyhedral distortions of the AlO₆ octahedra.